Perfluorinated Trialkoxysilanol with Dramatically Increased Bronsted Acidity.
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ABSTRACT:
SUBMITTER: Feige F
PROVIDER: S-EPMC9292728 | biostudies-literature |
REPOSITORIES: biostudies-literature
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Project description:Understanding the adsorption behavior of base probes in aluminosilicates and its relationship to the intrinsic acidity of Brønsted acid sites (BAS) is essential for the catalytic applications of these materials. In this study, we investigated the adsorption properties of base probe molecules with varying proton affinities (acetonitrile, acetone, formamide, and ammonia) within six different aluminosilicate frameworks (FAU, CHA, IFR, MOR, FER, and TON). An important objective was to propose a robust criterion for evaluating the intrinsic BAS acidity (i.e., state of BAS deprotonation). Based on the bond order conservation principle, the changes in the covalent bond between the aluminum and oxygen carrying the proton provide a good description of the BAS deprotonation state. The ammonia and formamide adsorption cause BAS deprotonation and cannot be used to assess intrinsic BAS acidity. The transition from ion-pair formation, specifically conjugated acid/base interaction, in formamide to strong hydrogen bonding in acetone occurs within a narrow range of base proton affinities (812-822 kJ mol-1). The adsorption of acetonitrile results in the formation of hydrogen-bonded complexes, which exhibit a deprotonation state that follows a similar trend to the deprotonation induced by acetone. This allows for a semi-quantitative comparison of the acidity strengths of BAS within and between the different aluminosilicate frameworks.
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Project description:We have designed and realized an efficient and operationally simple single-flask synthesis of imidodiphosphate-based Brønsted acids. The methodology proceeds via consecutive chloride substitutions of hexachlorobisphosphazonium salts, providing rapid access to imidodiphosphates (IDP), iminoimidodiphosphates (iIDP), and imidodiphosphorimidates (IDPi). These privileged acid catalysts feature a broad acidity range (pKa from ∼11 to <2 in MeCN) and a readily tunable confined active site. Our approach enables access to previously elusive catalyst scaffolds with particularly high structural confinement, one of which catalyzes the first highly enantioselective (>95:5 er) sulfoxidation of methyl n-propyl sulfide. Furthermore, the methodology delivers a novel, rationally designed super acidic catalyst motif, imidodiphosphorbis(iminosulfonylimino)imidate (IDPii), the extreme reactivity of which exceeds commonly employed super-Brønsted acids, such as trifluoromethanesulfonic acid. The unique reactivity of one such IDPii catalyst has been demonstrated in the first α-methylation of a silyl ketene acetal with methanol as the electrophilic alkylating reagent.
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Project description:A series of unique tunable aryl-imidazolium magnetic ionic liquids (MILs) with dual acidity that contain both Brønsted and Lewis acidic sites (abbreviated as B-L MILs) were synthesized and characterized using nuclear magnetic resonance and mass spectrometry. Physical properties, such as thermal properties, magnetic susceptibility, and Brønsted and Lewis acidity, were measured. These properties were found to depend on the cation structure. These B-L MILs had good solubility in many organic solvents, good thermal stability, and low melting points, and exhibited magnet-like behavior. For these B-L MILs, the Brønsted acidity was measured using ultraviolet-visible (UV-Vis), and the Lewis acidity was measured using Fourier transform infrared spectroscopy (FTIR). The results showed that B-L MILs with an electron-withdrawing group in the aryl-imidazolium moiety had higher Brønsted acidity, whereas those with an electron-donating group had higher Lewis acidity. This type of ionic liquid, with both Brønsted and Lewis acidic sites, is expected to be a useful solvent and catalyst for organic reactions.
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Project description:The concept of hydrogen bonding for enhancing substrate binding and controlling selectivity and reactivity is central in catalysis. However, the properties of these key hydrogen bonds and their catalyst-dependent variations are extremely difficult to determine directly by experiments. Here, for the first time the hydrogen bond properties of a whole series of BINOL-derived chiral phosphoric acid (CPA) catalysts in their substrate complexes with various imines were investigated to derive the influence of different 3,3'-substituents on the acidity and reactivity. NMR 1H and 15N chemical shifts and 1 J NH coupling constants of these hydrogen bonds were used to establish an internal acidity scale corroborated by calculations. Deviations from calculated external acidities reveal the importance of intermolecular interactions for this key feature of CPAs. For CPAs with similarly sized binding pockets, a correlation of reactivity and hydrogen bond strengths of the catalyst was found. A catalyst with a very small binding pocket showed significantly reduced reactivities. Therefore, NMR isomerization kinetics, population and chemical shift analyses of binary and ternary complexes as well as reaction kinetics were performed to address the steps of the transfer hydrogenation influencing the overall reaction rate. The results of CPAs with different 3,3'-substituents show a delicate balance between the isomerization and the ternary complex formation to be rate-determining. For CPAs with an identical acidic motif and similar sterics, reactivity and internal acidity correlated inversely. In cases where higher sterical demand within the binary complex hinders the binding of the second substrate, the correlation between acidity and reactivity breaks down.
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Project description:The stability of metal-organic frameworks (MOFs) typically decreases with an increasing number of defects, limiting the number of defects that can be created and limiting catalytic and other applications. Herein, we use a hemilabile (Hl) linker to create up to a maximum of six defects per cluster in UiO-66. We synthesized hemilabile UiO-66 (Hl-UiO-66) using benzene dicarboxylate (BDC) as linker and 4-sulfonatobenzoate (PSBA) as the hemilabile linker. The PSBA acts not only as a modulator to create defects but also as a coligand that enhances the stability of the resulting defective framework. Furthermore, upon a postsynthetic treatment in H2SO4, the average number of defects increases to the optimum of six missing BDC linkers per cluster (three per formula unit), leaving the Zr-nodes on average sixfold coordinated. Remarkably, the thermal stability of the materials further increases upon this treatment. Periodic density functional theory calculations confirm that the hemilabile ligands strengthen this highly defective structure by several stabilizing interactions. Finally, the catalytic activity of the obtained materials is evaluated in the acid-catalyzed isomerization of α-pinene oxide. This reaction is particularly sensitive to the Brønsted or Lewis acid sites in the catalyst. In comparison to the pristine UiO-66, which mainly possesses Brønsted acid sites, the Hl-UiO-66 and the postsynthetically treated Hl-UiO-66 structures exhibited a higher Lewis acidity and an enhanced activity and selectivity. This is further explored by CD3CN spectroscopic sorption experiments. We have shown that by tuning the number of defects in UiO-66 using PSBA as the hemilabile linker, one can achieve highly defective and stable MOFs and easily control the Brønsted to Lewis acid ratio in the materials and thus their catalytic activity and selectivity.
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Project description:The enhancing effect of extraframework Al (EFAl) species on the acidity of bridging hydroxyl groups in a steam-calcined faujasite zeolite (ultrastabilized Y, USY) was investigated by in situ monitoring the H/D exchange reaction between benzene and deuterated zeolites by 1H MAS NMR spectroscopy. This exchange reaction involves Brønsted acid sites (BAS) located in sodalite cages and supercages. In a reference faujasite zeolite free from EFAl, both populations of BAS are equally and relatively slowly reactive toward C6H6. In USY, in stark contrast, the H/D exchange of sodalite hydroxyl groups is significantly faster than that of hydroxyl groups located in the faujasite supercages, even though benzene has only access to the supercages. This evidences selective enhancement of BAS near Lewis acidic EFAl species, which according to the NMR findings are located in the faujasite sodalite cages.
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Project description:The influence of processing intumescent bio-based poly(lactic acid) (PLA) composites by injection and fused filament fabrication (FFF) was evaluated. A raw (ANa) and two acidic-activated (AH2 and AH5) montmorillonites were added to the intumescent formulation, composed by lignin and ammonium polyphosphate, in order to evaluate the influence of the strength and the nature (Brønsted or Lewis) of their acidic sites on the fire behavior of the composites. The thermal stability and the volatile thermal degradation products of the composites were assessed. The injected and 3D-printed composites were submitted to cone calorimeter (CC), limit oxygen index (LOI), and UL-94 flammability tests. A similar tendency was observed for the injected and 3D-printed samples. The high density of strong Lewis sites in AH2 showed to be detrimental to the fire-retarding properties. For the CC test, the addition of the intumescent composite reduced the peak of heat released (pHRR) in approximately 49% when compared to neat PLA, while the composites containing ANa and AH5 presented a reduction of at least 54%. However, the addition of AH2 caused a pHRR reduction of around 47%, close to the one of the composite without clay (49%). In the LOI tests, the composites containing ANa and AH5 achieved the best results: 39% and 35%, respectively, for the injected samples, and 35 and 38% for the 3D-printed samples. For the composite containing AH2 the LOI values were 34% and 32% for injected and 3D-printed samples, respectively. Overall, the best performance in the flammability tests was achieved by the composites containing clays with only weak and moderate strength acidic sites (ANa and AH5).
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